1. Field of the Invention
The present invention generally relates to the electronic industry and, more specifically, to radio-frequency transceiver systems. The present invention more specifically relates to a bi-directional coupler and to its applications.
2. Discussion of the Related Art
A coupler is generally used to draw part of the power present on a so-called main or primary transmission line towards another so-called coupled or secondary line located in the vicinity.
Couplers are distributed in two categories according to whether they are formed of discrete passive components (it is then spoken of couplers with local elements) or of conductive lines close to one another to be coupled (it is then spoken of couplers with distributed lines). The present invention relates to the second category of couplers.
In many applications, part of the power transmitted over a line needs to be sampled, for example, to control the power of amplifiers in a transmit circuit, to control the linearity of a transmit amplifier according to the losses due to the reflection of an antenna, to dynamically match an antenna, etc.
A coupler can be defined, among other things, by its directivity, which represents the power difference (expressed in dBm) between the two access ports of its coupled or secondary line.
Theoretically, an ideal coupler has an infinite directivity, that is, no power is present on the port of its secondary line located opposite to the output port of its main line when a signal flows on its main line from the input port to this output port. In practice, a coupler is said to be directional when its directivity is sufficient (typically greater than +20 dB) for the powers recovered on the access ports of its secondary line to enable making out the flow direction of the power in the main line. When the two ports of the coupler are used to simultaneously have the power information on the two ports of its secondary line, the coupler is said to be bi-directional.
If the two ports of its secondary line and the output port of its main line are perfectly matched, no parasitic reflection occurs. Such a perfect matching is difficult to obtain in practice. In particular, the port from which the power portion is sampled by coupling is seldom ideally matched. As a result, parasitic reflections generate errors on the recovered information.
A mismatch of the secondary line port of the coupler from which the information is sampled may have different sources. Most often, the coupler is placed on an insulating substrate (for example, of printed circuit type) to be associated with other circuits. It is then not possible to ensure a perfect matching (typically, at 50 ohms) of the measurement port.
To attempt overcoming this problem, it has already been provided to equip the ends of the secondary line with attenuators. However, at constant coupling factor, this requires increasing the coupling, and thus the coupler size, and thus increases transmission losses. Further, this only postpones the problem of parasitic reflections, which then appear for higher levels of mismatch of the secondary line ports.